Unmanned Aerial Vehicles (UAVs) have stepped out of the realm of academic research and military operations and are rapidly becoming useful in civilian applications. Advances in integrated avionics have driven the cost and complexity of UAVs down, and into the hands of commercial users. There is increasing demand for reliable UAV platforms that can carry large equipment payloads, and have longer flight times and range—particularly for indoor applications.
The utility of hovering UAV systems is bounded by available power sources, and the efficiency of transferring that energy to the air via rotors. This restricts the achievable payload, flight time and speed performance, typically to payloads of less than 1 kg or flight times of less than 20 minutes for vehicles small enough to fly through doorways and around human spaces. Increasing available energy stores correspondingly requires heavier (and thus larger) rotorcraft that would not fit in small spaces. The energy density of power sources improves slowly—thus, the current focus is on improving energetic performance of rotorcraft by maximising the power efficiency of lifting systems.
The power a rotorcraft requires to hover scales with the mass being lifted and the area of the rotor. For this reason, skycranes and cargo helicopters have particularly large diameter rotors or multiple rotors to reduce disc-loading. In contrast, the smaller rotors employed by quadrotors and multirotors are more energy intensive. However a trade-off of helicopters lies in the complex rotorhead mechanics, which are maintenance intensive—quadrotors, conversely, are simple and robust.
The small rotor sizes of quadrotors and multirotors makes them intrinsically less energy efficient than a traditional helicopter with a large single rotor. However, the quadrotor configuration's innate simplicity and inexpensive construction recommends its use in many aerial robotics applications.
Some prior art designs are aimed at increasing lift by employing rotors matched to a chemical fuel drive system; they rely on the drag torque of co-linear outboard rotors to provide counter-torque. Their use of numerous outboard rotors (four or more), which require a larger footprint for a given main rotor diameter, reduces the expected achievable efficiency compared with the proposed configuration.
There are two broad areas of interest in the performance of quadrotors: energetic performance and control performance. Energetics determines the achievable flight time, range and payload weight a rotorcraft can carry, while control determines its ability to manoeuvre and reject disturbances.
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